Diabetes mellitus is on the rise worldwide and is considered to be at an epidemic level by the World Health Organization. The clinical manifestation and progression of diabetes often vary considerably between countries and commonly between ethnic groups in the same country. Currently diabetes affects 151 million people worldwide and an estimate 300 million people in 2025. There are two main forms of diabetes. Type 1 (insulin-dependent diabetes mellitus, IDDM) is due primarily to autoimmune-mediated destruction of pancreatic β-cells, resulting in absolute insulin deficiency. It is the second most common chronic disease of children. By contrast, type 2 diabetes (non-insulin-dependent diabetes mellitus, NIDDM) is characterized by insulin-resistance and Inadequate insulin secretion. A significant fraction of individuals originally diagnosed with type 2 diabetes evolve with time to a type 1 state, which is defined as exhibiting anti-β-cell autoimmunity.
Because genetic factors contribute to the development of diabetes, the disease displays a strong familial aggregation. Although there are monogenic syndromes of insulin resistance, in which a definite gene has been identified as the cause of insulin resistance, these are relative rare. The more common presentation of diabetes appears to be polygenic. Additionally, behavioural- and lifestyle-related risk factors exist. Type 2 diabetes is Increasingly common primarily because of increases in the prevalence of a sedentary lifestyle and obesity. One of the major arguments for the role of behavioural factors in the etiology of diabetes has been the rapid increase in the prevalence and incidence of the disease in populations undergoing rapid westernization. The westernization transition is usually accompanied by increases in obesity, decreases in physical activity and alterations in dietary intake toward more calories, fat and non-complex carbohydrates.
Plasma glucose concentrations are normally maintained within a fairly narrow range despite wide fluctuations in the body's supply (e.g. meals) and demand (e.g. exercise) for nutrients. After an overnight fast, insulin-independent tissues, the brain (50%) and splanchnic organs (25%), account for most of the total body glucose disposal. Insulin-dependent tissues, adipose tissue and primarily skeletal muscles, are responsible for the remaining 25% of glucose utilization. This basal glucose uptake is precisely matched by the release of glucose from the liver. In response to hyperglycemia after a meal, pancreatic insulin secretion is stimulated and the combination of hyperinsulinemia plus hyperglycemia promotes glucose uptake (by splanchnic and peripheral, primarily muscle, tissues) and suppresses hepatic glucose production. It follows, therefore, that defects at the level of the β-cell, muscle and liver can lead to the development of glucose intolerance and diabetes mellitus. All the abnormalities in diabetes basically result from an imbalance between insulin sensitivity and insulin secretion. The initial stage of diabetes is characterised by impaired glucose tolerance and postprandial hyperglycemia. As the disease progresses, fasting hyperglycemia is observed.
The earliest detectable abnormality in NIDDM is an impairment in the body's ability to respond to insulin. Because the pancreas is able to appropriately augment its secretion of insulin to offset the insulin resistance, glucose tolerance remains normal. With time, however, the beta-cell fails to maintain its high rate of insulin secretion and the insulin resistance leads to the development of impaired glucose tolerance and eventually overt diabetes mellitus. The cause of pancreatic “exhaustion” remains unknown. Insulin resistance in NIDDM involves both hepatic and peripheral tissues. In response to both endogenously secreted or exogenously administered insulin, hepatic glucose production fails to suppress normally and muscle glucose uptake is diminished. The accelerated rate of hepatic glucose output is due mainly to augmented gluconeogenesis. In muscle many cellular defects in insulin action have been described including impaired insulin-receptor tyrosine kinase activity, diminished glucose transport, and reduced glycogen synthase and pyruvate dehydrogenase activities. The abnormalities account for disturbances in the two major intracellular pathways of glucose disposal, glycogen synthesis and glucose oxidation. In the earliest stages of NIDDM, the major defect involves the inability of insulin to promote glucose uptake and storage as glycogen. Other potential mechanisms that have been put forward to explain the glucose intolerance include increased levels of free fatty acids, chronic low-grade activation of the immune system (increased levels of TNFα and IL6), altered skeletal muscle blood flow, increased conversion of amylin to its insoluble amyloid form and glucose toxicity.
Diabetes is associated with a variety of physiologic disorders such as hypertension and dyslipidemia. Diabetes also increases the risk of macrovascular (coronary artery disease, stroke, amputation) and microvascular (blindness, renal failure, neuropathies) diseases. Myocardial infarction, stroke or renal failure are the cause of death for more than 70% of diabetes patients. The huge mortality and debilitating neuropathies associated with diabetes underline the importance of active medical intervention.
There are several ways to counteract diabetes. The first is lifestyle adjustments aimed at improving endogenous insulin sensitivity. This can be achieved by increased physical activity and bodyweight reduction with diet and behavioural modification. Unfortunately, most people with non-insulin-dependent diabetes mellitus never receive sufficient nutritional education or are not capable of complying with a strict diet regimen.
Another therapeutic way involves increasing insulin availability by the administration of exogenous insulin, insulin analogues and insulin secretagogues such as sulphonylureas. The primary mode of action of sulphonylureas is through the depolarisation of the pancreatic β-cells by blocking the ATP-dependent potassium channels and causing an influx of calcium ions, which stimulate insulin secretion. The most frequently encountered adverse effect of insulin, insulin analogues and insulin secretagogues is hypoglycemia. Bodyweight gain can also be a concern, because insulin not only increases uptake of blood glucose but also promotes the synthesis and storage of lipids.
Biguanides, of which metformin is the most commonly used, also have proven to be effective anti-hyperglycemic agents. Metformin reduces hepatic gluconeogenesis and basal hepatic glucose output. Its most serious adverse effect is lactic acidosis. Other common adverse effects of metformin are nausea and anorexia. Oral antidiabetics such as sulphonylureas and metformin as monotherapy or in combination have been shown to decrease fasting plasma glucose levels, but postprandial hyperglycemia persists in more than 60% of patients and probably accounts for sustained increases of hemoglobin A1c levels.
α-Glucosidase inhibitors, e.g. acarbose and miglitol, primarily target postprandial hyperglycemia. The therapy of diabetes mellitus with α-glucosidase inhibitors is based on a delayed intestinal degradation of starch and sucrose. These carbohydrates must be hydrolysed by α-glucosidases to monosaccharides before they can be transported through the mucosa of the small intestine. The reversible inhibition of the brush border glucosidases results in redistribution of carbohydrate absorption from the upper portion of the gut to a more extended surface area covering the whole length of the small intestine. This is accompanied by a delayed absorption of monosaccharides and a decrease in the postprandial elevation of blood glucose. Common adverse effects of α-Glucosidase inhibitors are symptoms of carbohydrate malabsorption and gastrointestinal discomfort.
Another class of antidiabetic drugs are thiazolidinediones, such as rosiglitazone and pioglitazone, which are insulin sensitizers and act through activation of peroxisome proliferator-activated receptor γ (PPARγ). PPARγ is mainly expressed in adipose tissues, plays an important role in adipogenesis and modifies fatty acid synthesis and storage. Binding of rosiglitazone to PPARγ results in reduced endogenous glucose production and increased blood glucose uptake. It increases the sensitivity of skeletal muscle, liver and adipose tissues to insulin. Improvements in glucose metabolism with rosiglitazone treatment are closely correlated with decreased plasma free fatty acid metabolism. The stimulation by rosiglitazone of PPARγ in adipose tissue and subsequent adipocyte differentiation results in the generation of more, but smaller, adipocytes which are more insulin sensitive and produce less free fatty acid, TNFα and leptin. Common adverse effects of rosiglitazone are anemia, oedema and increased body weight.